1. Field of the Invention
The present invention relates generally to a method of driving a piezo-electric type ink jet head for jetting an ink out of a nozzle by making use of a distortion of a piezo-electric element and, more particularly, to a method of driving a piezo-electric type ink jet head for changing a quantity of ink particles jetted out.
2. Description of the Related Art
Ink jet printers are used for apparatuses such as a printer, facsimile and so on. There has been availed a piezo-electric type ink jet printer using a piezo-electric element among those ink jet printers. The piezo-electric type ink jet printer is constructed to jet inks out of a nozzle by making use of a distortion of the piezo-electric element.
In this type of ink jet printer, a print dot diameter is required to be variable to express gradations of a print. For this purpose, changing a quantity of ink particles to be jetted is requested of the above printer.
A method of jetting the inks is classified into a positive polarity drive method of sucking the inks after jetting out the inks, and a negative polarity drive method of jetting out the inks after sucking the inks. According to the negative polarity drive method, a scatter of the ink particles is stable, and a possible-of-getting-particled frequency is broad.
FIGS. 25A-25D and FIGS. 26A-26E are explanatory diagrams showing a first prior art.
A d31-mode is a mode making most of a distortion caused when the piezo-electric element shrinks upon an application of a positive voltage. In this mode, the piezo-electric element is distorted in a perpendicular direction for an electric-field direction. In this d31-mode, when a voltage indicated by a dotted line in FIG. 25A is applied to the piezo-electric element, operation of jetting the inks is performed after sucking the inks.
FIGS. 26A-26E are enlarged views of the nozzle. A meniscus 10 is formed in a nozzle 1. Herein, a velocity vector the meniscus has is expressed by "V".
FIG. 26A shows a state of how the nozzle 1 and the meniscus 10 might be when the piezo-electric element is in an initial state. A surface tension of the meniscus 10 equibrates with a negative within the pressure chamber, and the meniscus 10 exists in an initial position in the vicinity of a nozzle outlet.
FIG. 26B shows a position. of the meniscus 10 when increasing the negative pressure in the pressure chamber by letting the piezo-electric element shrink in such a direction as to expand the pressure chamber. That is, it shows a case where a positive voltage having a positive inclination is, as indicated by a dotted line in FIG. 25A, applied thereto. The negative pressure in the pressure chamber gets larger than the surface tension of the meniscus 10, and the meniscus 10 is receded toward the pressure chamber.
FIG. 26C shows a position when the negative pressure in the pressure chamber is reduced due to an influx of the inks from an ink supply port enough to reduce the negative pressure in the pressure chamber and the meniscus 10 is substantially stopped. At this time, the meniscus 10 is forced to move back to the vicinity of the pressure chamber.
FIG. 26D shows a position of the meniscus 10 when the piezo-electric element is abruptly expanded in such a direction as to contract the pressure chamber. That is, it shows a case where a voltage having a negative inclination is, as indicated by a dotted line in FIG. 25A, applied thereto. The meniscus 10 forms a layer flow by dint of a positive pressure within the pressure chamber and the surface tension of the meniscus, and has a larger velocity toward the nozzle outlet. Accordingly, the meniscus 10 quickly moves toward the nozzle outlet.
FIG. 26E shows a state of the meniscus 10 when the expansion of the piezo-electric element is stopped. The pressure in the pressure chamber becomes a large negative pressure due to a flux of the inks to the ink supply port as well as to the nozzle 1. Therefore, the inks in the nozzle 1 are abruptly decelerated. The ink liquid outside the nozzle has, however, a velocity enough to scatter out and hence overwhelms the surface tension given from the inks of the nozzle 1, thereby getting particled. Thereafter, the inks having an insufficient velocity are forced to return inside the nozzle 1 by dint of the surface tension.
The states described above are repeated, thereby forming the ink particles and jetting them out.
Known as a first prior art method of controlling the particle quantity of the ink particles is a method of decreasing a voltage amplitude applied to the piezo-electric element down to V2 as indicated by a solid line in FIG. 25A. The particle quantity of the ink particles can be reduced by this method. FIGS. 25B through 25D show a state of how the nozzle 1 and the meniscus 10 might be when jetting the small ink particles.
FIG. 25B shows a state when starting a suction of the inks. The meniscus 10 is moving toward the pressure chamber.
FIG. 25C shows a state of how the nozzle 1 and the meniscus 10 might be when finishing the suction of the inks and starting the jet-out of the inks. Since an amplitude of the voltage applied to the piezo-electric element is reduced, a recession quantity of the meniscus becomes smaller than in the case of FIG. 26c.
FIG. 25D shows a state of how the nozzle 1 and the meniscus 10 might be when the ink liquid gets particled. As the recession quantity of the meniscus 10 has been decreased, the ink particle quantity also decreases.
A second prior art method of controlling the ink particle quantity will be explained with reference to FIGS. 27A through 27D.
According to the second method, the ink particle quantity is reduced by changing a receding velocity of the meniscus. Along with this, a jetting speed is controlled. More specifically, as indicated by a solid line in FIG. 27A, a drive voltage for the piezo-electric element remains unchanged, and a rising slope of the drive voltage is made steep. The particle quantity of the ink particles becomes smaller as this slope gets steeper and steeper. Note that the drive waveform in the case of generating a normal quantity of ink particles is shown by the dotted line as in FIG. 25A.
FIG. 27B shows a state of how the nozzle 1 and the meniscus 10 might be when starting the suction of the inks. At this time, a higher velocity toward the pressure chamber is given to the meniscus 10 by quickly sucking the inks than in the case of jetting a normal quantity of the ink particles (as indicated by the dotted line in FIG. 27A). Then, the meniscus 10 is forced to move to the vicinity of the pressure chamber.
FIG. 27C shows a state of how the nozzle 1 and the meniscus 10 might be when starting the jet-out of the inks upon finishing the suction of the inks. The meniscus 10 is receded to the vicinity of the pressure chamber in the nozzle 1 by the suction of the inks, and therefore the inks can be accelerated enough.
FIG. 27D shows a state of how the nozzle 1 and the meniscus 10 might be when the ink liquid gets particled. The ink liquid having the sufficient velocity gets particled and thereafter scatter out.
A third prior art method of controlling the ink particle quantity will be described referring to FIGS. 28A through 28D.
According to the third method, as indicated by a solid line in FIG. 28A, the drive voltage is reduced down to V2 as in the first prior art method, and the recession quantity of the meniscus when sucking the inks is decreased. Along with this, a voltage changing velocity when jetting the inks is made much higher, thereby preventing the meniscus from decreasing in its velocity when jetting the inks.
FIG. 28B shows a state of how the nozzle and the meniscus might be when starting the suction of the inks. FIG. 28C shows a state of how the nozzle and the meniscus might be when finishing the suction of the inks. The voltage amplitude is reduced, and hence the meniscus 10 is not receded to the vicinity of the pressure chamber. Herein, as described above, the inks are quickly jetted out. At this moment, the inks in close proximity to the nozzle outlet are jetted out while being incapable of obtaining the sufficient velocity. Those inks are, however, mixed with forthcoming inks that are enough accelerated and become the ink particles having a desired velocity on the whole.
FIG. 28D shows a state of how the nozzle and the meniscus might be when the ink liquid gets particled. The sufficiently accelerated ink liquid get particled and thereafter scatter out.
The third method is intended to compensate a drop in the velocity that is caused due to the reduction in the recession quantity of the meniscus.
There arise, however, the following problems inherent in the first prior art method.
The ink liquid is, while existing in the nozzle, pushed by the positive pressure in the pressure chamber and can be accelerated. Once the ink liquid is jetted out of the nozzle outlet, however, the ink liquid can not be accelerated much higher than it. Therefore, as done by this method, if the recession quantity of the meniscus 10 is reduced, some ink liquid existing in the vicinity of the nozzle outlet in the intra nozzle ink liquid is jetted out of the nozzle outlet while not being sufficiently accelerated.
For this reason, the ink liquid does not reach a target velocity and is not accelerated. Thereafter, the ink liquid, which has not been accelerated, is mixed with the forthcoming ink liquid having the sufficient velocity. However, the layer flow state disappears, and hence the direction of a velocity vector of the ink particles is disturbed. This leads to a decline in terms of a scatter stability. In combination with this, a kinetic energy is lost by the mixture of the ink liquids, whereby the average ink particle velocity slows down. This might cause a disturbance in a printed image.
Further, the second prior art method presents the problems which follow.
Just when the meniscus 10 is abruptly receded, the pressure in the pressure chamber is changed to the positive pressure, and therefore, as shown in FIG. 27D, a velocity distribution in radial direction of the nozzle is disturbed. This results in a disturbance in the scattering direction of the ink particles. Therefore, in the drive waveform shown in FIG. 27A, a time Trb can not be shortened enough, so that a variation width of the particle quantity of the ink particles can not be taken large.
Moreover, the third prior art method has the following problems.
(1) As in the case of the first prior art method, since the recession quantity of the meniscus is reduced, the scattering direction of the ink particles is disturbed.
(2) An increase in the variation width of the particle quantity of the ink particles entails a quick enhancement of the velocity of the meniscus when jetted out. Even if the velocity of the meniscus when jetted out is quickly increased, however, the velocity of the meniscus when jetted out is restricted by a natural frequency of the piezo-electric element. Therefore, the variation width of the particle quantity of the ink particles can not be taken large.
(3) If the velocity of the meniscus when jetted out is quickly increased, an overshoot of the piezo-electric element enlarges, and a large quantity of satellite particles are produced. This might cause a decline in terms of the print quality, with result that the variation width of the particle quantity of the ink particles can not be taken large.